See below links to several sources of data for your air quality modeling projects:
WebMET.com provides free meteorological data for both the United States and Canada! The site not only includes free met data for air dispersion models, but links to meteorological books, videos, software and more. Explore and Learn using WebMET for all things meteorological.
WebGIS.com contains extensive map coverage for technical applications including Air Dispersion Modeling, Human Health & Ecological Risk Assessment and Terrain Processing for free! These maps are ready for download in various formats, to facilitate your modeling.
SRTM - Shuttle Radar Topography Mission - Terrain data in resolutions of 90m and 900m covering the entire world and of 30m for the US (FREE):SRTM1 (U.S. Coverage ~30m)
SRTM3 (Global Coverage ~90m)
SRTM30 (Global Coverage ~900m)